Method and measuring system for ascertaining density of a fluid

ABSTRACT

A method for producing at least one oscillation measurement signal, which has vibrations of a vibratory body are registered. A temperature sensor is applied thermally attached with a non fluid contacting, second surface of the vibratory body for producing a temperature measurement signal representing a time curve of a variable temperature of the vibratory body. The temperature measurement signal can follow, however time delayed, a change of the temperature of the vibratory body from a beginning temperature value, to a new temperature value. Based on the oscillation measurement signal as well as the temperature measurement signal, density, measured values are produced representing the density, wherein, during such, discrepancies possibly occurring between the time curve of the temperature of the vibratory body and the temperature measurement signal are taken into consideration, respectively at least partially compensated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 13/716,637filed Dec. 17, 2012, which claims the priority date of German PatentApplication 10 2011 089 808.5 filed on Dec. 23, 2011, the content ofwhich is hereby incorporated by reference into this application.

TECHNICAL FIELD

The invention relates to a method for ascertaining density, p, of afluid contacting an oscillatably held, vibratory body, which can beexcited to execute vibrations. The invention relates as well as to acorresponding measuring system suitable for performing the method.

BACKGROUND DISCUSSION

Used often in industrial process measurements technology forascertaining density of fluid flowing in a pipeline or stored in acontainer are measuring systems, in the case of which an oscillatablyheld, vibratory body, as part of a physical to electrical, measuringtransducer, is brought into contact with the fluid to be measured,namely with a volume portion thereof. The vibratory body—contacted byfluid—is caused during operation to vibrate in such a manner, forexample, actively by means of an electro-mechanical oscillation exciteracting on the vibratory body, that the vibratory body executes, at leastpartially, resonant oscillations, namely mechanical oscillations with aresonant frequency, which is dependent on the mechanical construction ofthe vibratory body, as well as also on the density of the fluid. Themeasuring transducer is, for such purpose, most often applied in acontainer wall of the container, for example, a tank, holding the fluidor in the course of a line, for example, a pipeline, through which thefluid is moving, and is equipped also to register vibrations of thevibratory body and to produce at least one oscillation measurementsignal, which has at least one signal component with a signal frequencycorresponding to the resonant frequency and, consequently, dependent onthe density of the fluid. Examples of such measuring transducers,respectively measuring systems, formed by means of one or more vibratorybodies and thus suitable for measuring density, are described in, amongothers, EP-A 564 682, EP-A 919 793, US-A 2007/0028663, US-A200810127745, US-A 2010/0083752, US-A 2010/0236323, US-A 2011/0219872,U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,801,897, U.S. Pat. No.5,027,662, U.S. Pat. No. 5,054,326, U.S. Pat. No. 5,796,011, U.S. Pat.No. 5,965,824, US-A 60 73495, U.S. Pat. No. 6,138,507, U.S. Pat. No.6,148,665, U.S. Pat. No. 6,044,694, U.S. Pat. No. 6,389,891, U.S. Pat.No. 6,651,513, U.S. Pat. No. 6,688,176, U.S. Pat. No. 6,711,942, U.S.Pat. No. 6,845,663, U.S. Pat. No. 6,912,904, U.S. Pat. No. 6,938,475,U.S. Pat. No. 7,040,179, U.S. Pat. No. 7,102,528, U.S. Pat. No.7,272,525, U.S. Pat. No. 7,549,319, U.S. Pat. No. 7,681,445, U.S. Pat.No. 7,874,199, WO-A 00/19175, WO-A 01/02816, WO-A 01/29519, WO-A88/02853, WO-A 93/01473, WO-A 93/19348, WO-A 93/21505, WO-A 94/21999,WO-A 95/03528, WO-A 95/16897, WO-A 95/29385 or WO-A 98/02725. Inaccordance therewith, the vibratory body can be e.g. a measuring tubeinserted into the course of the line carrying the fluid, thus ameasuring tube through which the fluid is flowing—for instance, themeasuring tube of a measuring transducer of a measuring system in theform of a purely density measuring device for flowing fluids, in theform of a Coriolis, mass flow/-density measuring device and/or in theform of a density-/viscosity measuring device—or, however, e.g. also avibratory body formed by means of an oscillating cylinder extending intothe fluid—located in a line or in a container—and formed, in givencases, in rod- or paddle shape and/or internally hollow, consequentlyprovided e.g. also by a vibronic fill level limit switch measuring alsodensity, supplementally to a limit of fill-level.

The measuring transducer is, furthermore, connected with an electronicsof the measuring system serving for evaluation of the at least oneoscillation measurement signal and for generating corresponding density,measured values representing the density. In the case of modernmeasuring systems of the type being discussed, such electronics are, asdescribed in, among others, also in U.S. Pat. No. 6,311,136 or U.S. Pat.No. 6,073,495, most often implemented by means of one or moremicroprocessors formed, in given cases, also as digital signalprocessors (DSP). Besides evaluation of the at least one oscillationmeasurement signal delivered by the measuring transducer andrepresenting oscillations of its vibratory body, the electronics servesalso to generate at least one driver signal, for example, an harmonicand/or clocked, driver signal, for an electro-mechanical oscillationexciter acting on the vibratory body and serving for actively excitingsaid oscillations, for example, an electro-mechanical oscillationexciter having an exciter coil interacting with a permanent magnetaffixed on the vibratory body or a piezoelement affixed on the vibratorybody, wherein the driver signal has a signal component with a signalfrequency matched to the resonant frequency of the vibratory body. Thesignal component, respectively the driver signal, can, for example, alsobe controlled as regards its electrical current level and/or voltagelevel.

In the case of measuring systems of the type being discussed, theelectronics is most often accommodated within at least one,comparatively robust, especially impact-, pressure-, and/or weatherresistant, electronics housing. The electronics housing can be arranged,for example, remotely from the measuring transducer and connectedtherewith only via a flexible cable; it can, however, also be arranged,as shown e.g. also in the initially mentioned U.S. Pat. No. 5,796,011,directly on the measuring transducer or on a measuring transducerhousing separately housing the measuring transducer, and thus, also, itsvibratory body. Moreover, however, as shown in, among others, WO-A01/29519, it is also quite usual, in given cases, to use modularlyformed electronics accommodated in two or more separate housing modulesfor forming measuring systems of the type being discussed.

In the case of measuring systems of the type being discussed, theelectronics is usually electrically connected via correspondingelectrical lines to a superordinated electronic data processing systemmost often arranged spatially removed from the respective device. Mostoften, the electronic data processing system is also spatiallydistributed. Measured values produced by the respective measuring systemare forwarded near in time by means of a measured value signalcorrespondingly carrying the measured values. Measuring systems of thetype being discussed are additionally usually connected with one anotherby means of a data transmission network provided within thesuperordinated data processing system and/or with correspondingelectronic process controls, for example, on-site programmable logiccontrollers or process control computers installed in a remote controlroom, where the measured values produced by means of the respectivemeasuring system and digitized and correspondingly coded in suitablemanner are forwarded. By means of such process control computers, thetransmitted measured values can be further processed and visualized ascorresponding measurement results e.g. on monitors and/or converted intocontrol signals for other field devices embodied as actuating devices,such as e.g. magnetically operated valves, electric-motors, etc. Sincemodern measuring arrangements can most often also be monitored and, ingiven cases, controlled and/or configured directly from such controlcomputers, operating data intended for the measuring system are equallydispatched in corresponding manner via the aforementioned datatransmission networks, which are most often hybrid as regards thetransmission physics and/or the transmission logic. Accordingly, thedata processing system serves usually also, to condition the measuredvalue signal delivered by the measuring system in accordance with therequirements of downstream data transmission networks, for example,suitably to digitize the measured value signal and, in given cases, toconvert such into a corresponding telegram, and/or to evaluate suchon-site. For this purpose, there are provided in the data processingsystems, electrically coupled with the respective connecting lines,evaluating circuits, which pre- and/or further-process as well as, incase required, suitably convert, the measured values received from therespective measuring system. Serving for data transmission in suchindustrial data processing systems are at least sectional, especiallyserial, fieldbusses, such as e.g., FOUNDATION FIELDBUS, RACKBUS—RS 485,PROFIBUS, etc., or, for example, also networks based on the ETHERNETstandard, as well as the corresponding, most often comprehensivelystandardized, transmission protocols. Alternatively or supplementally,in the case of modern measuring systems of the type being discussed,measured values can also be transmitted wirelessly per radio to therespective data processing system.

Besides the evaluating circuits required for processing and convertingthe measured values delivered from the respectively connected measuringsystem, such superordinated data processing systems have most often alsoelectrical supply circuits serving for supplying the connectedmeasuring- and/or switching devices with electrical energy. Suchelectrical supply circuits provide a supply voltage for the respectiveelectronics and drive the electrical currents flowing through electricallines connected thereto as well as through the respective electronics.In given cases, such voltage is fed directly from the connectedfieldbus, A supply circuit can, in such case, be associated with, forexample, exactly one measuring system, respectively a correspondingelectronics, and can be accommodated, together with the evaluatingcircuit associated with the respective measuring system —, for example,united into a corresponding fieldbus adapter—in a shared electronicshousing, e.g. in the form of a hatrail module. It is, however, alsoquite usual to accommodate supply circuits and evaluating circuits, ineach case, in separate electronics housings, in given cases, spatiallyremote from one another and to wire them together via external lines.

In the case of measuring systems for density measurement, wherein themeasuring system operates by means of a vibratory body, such asdisclosed in, among others, the initially mentioned WO-A 88/02853, WO-A98102725, WO-A 94/21999, in the case of ascertaining the density ρ basedon the resonant oscillations of the vibratory body, or its resonantfrequency f_(r), the temperature Θ₁₀ of the vibratory body, thus atemperature of the vibratory body dependent on a temperature of thefluid to be measured, consequently a variable temperature of thevibratory body, is to be take into consideration. For ascertaining such,a local temperature Θ_(sens) of the vibratory body on a surface of thevibratory body facing away from the fluid, consequently a “dry” surfacenot contacted thereby, is registered by sensor, usually by means of athereon adhered, platinum resistor of a resistance thermometer or bymeans of a thermocouple adhered on said surface, as well as acorresponding measuring circuit in the electronics. Such temperature isthen correspondingly taken into consideration in ascertaining thedensity, for instance, according to the relationships, Θ_(sens)˜Θ₁₀,f_(r) ²=f(Θ_(sens)→Θ₁₀), or f_(r) ²=f(1/φ. An additional improvement ofthe accuracy, with which the density can ultimately be measured, can beachieved in the case of measuring systems of the type being discussed,not least of all in the case of such having as vibratory body ameasuring tube clamped on its two ends, in among other ways, byregistering, as mentioned, among others, also in US-A 2011/0219872,furthermore, mechanical deformations of the vibratory body located inits static rest position, for instance, deformations as a result of achanging temperature of the vibratory body and/or as a result of forcesacting on the vibratory body, or mechanical stresses within thevibratory body resulting therefrom and correspondingly taking such intoconsideration in calculating the density. Such mechanical deformationsof the vibratory body can be registered, for example, by means of one ormore strain sensors mechanically coupled with the vibratory body via its“dry” surface.

Further investigations on measuring systems of the type being discussedhave, however, shown that, based on the measured temperature Θ₁₀ andresonant frequency f_(r), the density ρ of fluids can indeed be veryexactly ascertained, namely directly with a relative measuring error ofless than 0.2%, in cases where temperature remains constant over longerperiods of time of several minutes or more. However, especially in thecase of a change of the fluid in the line, the density measured for the“new” fluid can, first of all, deviate considerably from its actualdensity; this—even in the case of application of strainsensors—unluckily, at times, even in such a manner that, in the case ofa fluid with a density actually reduced relative to the preceding fluid,first of all, a higher density than earlier is ascertained, respectivelyalso, conversely, in spite of greater density for the “new” fluid, firstof all, a lesser density is ascertained. Consequently, the measuringerror for the density has, in comparison to its change, an oppositesign, respectively, the measuring system has, insofar, an all-passcharacteristic.

SUMMARY OF THE INVENTION

Taking this into consideration, it is an object of the invention toprovide a method for ascertaining density of a fluid by means of avibratory body contacted thereby, which method is successful alsodirectly after said fluid is introduced into the vibratory body asreplacement for another fluid. The method should work as much aspossible while using conventional vibratory bodies, respectively, whileusing conventional sensor arrangements serving for registering thetemperature of the vibratory body.

For achieving the object, the invention resides in a method forascertaining density of a fluid contacting an oscillatably held,vibratory body excitable to execute vibrations, for example, a vibratorybody of metal, wherein the vibratory body has a specific thermalconductivity of, for example, greater than 5 W K⁻¹ m⁻¹, thus a thereondependent, thermal conductance effective for heat transfer from, on theone hand, a fluid contacting, first surface of the vibratory body, whichhas a fluid temperature, namely a temperature of the fluid contactingthe first surface, to, on the other hand, a non fluid contacting, secondsurface, as well as having a heat capacity, and wherein a temperature ofthe vibratory body, namely a temperature of the vibratory body dependenton the fluid temperature, is variable. The method comprises steps ofcausing the vibratory body contacted by fluid to vibrate in such amanner that it executes, at least partially, resonant oscillations,namely mechanical oscillations with a resonant frequency dependent onthe density of the fluid contacting the first surface of the vibratorybody as well as also on the temperature of the vibratory body, as wellas registering vibrations of the vibratory body for producing at leastone oscillation measurement signal, which has at least one signalcomponent with a signal frequency corresponding to the resonantfrequency and consequently dependent on the density of the fluid,applying a temperature sensor thermally coupled with the vibratory bodyvia its second surface for producing a temperature measurement signalrepresenting a time curve of a temperature of the vibratory body, namelya temperature of the vibratory body dependent on a temperature of thefluid contacting the vibratory body on its first surface, wherein thetemperature measurement signal, not least of all as a result of thethermal conductance and the heat capacity of the vibratory body, followsa change of the temperature of the vibratory body from a beginning firsttemperature value to a second temperature value only time delayed, achange resulting, for example, from a change of the temperature of thefluid contacting the vibratory body on its first surface and/or from afluid change, so that the temperature measurement signal correspondsconsequently to said second temperature value only time delayed.Furthermore, the method of the invention comprises a step of producing ameasured value of density based on the oscillation measurement signal aswell as the temperature measurement signal during a change of thetemperature of the vibratory body resulting, for example, from a changeof the temperature of the vibratory body on its first surface. Indeed,the measured value of density is produced in such a manner that adiscrepancy, especially a time dependent discrepancy, occurring duringthe producing of the measured value of density between the time curve ofthe temperature of the vibratory body and the temperature measurementsignal, is taken into consideration, for example, even at leastpartially compensated.

Furthermore, the invention resides in a measuring system forascertaining density of a fluid, for example, a fluid flowing in apipeline, which measuring system comprises a measuring transducer havingat least one vibratory body, for example, a vibratory body of metal,which is oscillatably held and adapted to be contacted on a firstsurface by fluid to be measured in such a manner that the first surfaceassumes a fluid temperature, namely a temperature of the fluidcontacting the first surface, and to be caused to vibrate in such amanner that it executes, at least partially, resonant oscillations,namely mechanical oscillations with a resonant frequency dependent onthe density of the fluid, and which has a specific thermal conductivity,λ₁₀, for example, of greater than 5 W K⁻¹ m⁻¹, consequently a therefromdependent, effective thermal conductance, Λ₁₀, for heat transfer fromthe first surface to a non fluid contacting, second surface, and a heatcapacity, C₁₀, and having at least one oscillation sensor forregistering vibrations of the measuring tube and for producing anoscillation measurement signal, which has at least one signal componentwith a signal frequency dependent on the density of the fluid, andhaving a temperature sensor thermally coupled with the second surface ofthe vibratory body for registering a temperature on the second surfaceof the vibratory body dependent on the fluid temperature, and forproducing a temperature measurement signal representing a time curve ofa temperature of the vibratory body, namely a temperature of thevibratory body dependent on the fluid temperature, wherein thetemperature measurement signal, not least of all caused through thethermal conductance, Λ₁₀, and the heat capacity, C₁₀, of the vibratorybody, follows a change of the temperature of the vibratory body from abeginning first temperature value to a second temperature value, onlytime delayed, for instance, a change of the temperature resulting from achange of the temperature of the fluid contacting the vibratory body onits first surface and/or a change of the fluid, so that the temperaturemeasurement signal corresponds consequently to said second temperaturevalue, only time delayed. The measuring system further comprises anelectronics electrically connected with the measuring transducer forprocessing the oscillation measurement signal and the temperaturemeasurement signal as well as for generating, based on both theoscillation—as well as also the temperature measurement signal, ameasured value of density representing the density of the fluid. Theelectronics of the measuring system of the invention is, furthermore,arranged, during the generating of the measured value of density, totake into consideration a discrepancy occurring between the time curveof the temperature of the vibratory body and the temperature measurementsignal, especially a time dependent discrepancy, especially in such amanner that said discrepancy is at least partially compensated.

According to a first embodiment of the method of the invention, suchfurther comprises a step of applying the oscillation measurement signalfor producing a measured value of frequency representing the resonantfrequency of the vibratory body contacted by the fluid. Furthermore, themethod comprises a step of applying the temperature measurement signalfor producing a measured value of temperature representing thetemperature of the vibratory body as well as a step of applying both themeasured value of frequency as well as also the measured value oftemperature for producing a measured value of density representing thedensity.

According to a second embodiment of the method of the invention, suchfurther comprises a step of producing a sampled sequence of frequency,namely a sequence of digital frequency values ascertained at differentpoints in time based on the at least one oscillation measurement signal.Such sequence approximates a time curve of the resonant frequency of thevibratory body. Developing this embodiment of the invention further, itis additionally provided that the sampled sequence of frequency isapplied for producing a delayed sampled sequence of frequency, namely asequence of digital frequency values ascertained at different points intime based on the sampled sequence of frequency, in order to approximatethe time curve of the resonant frequency of the vibratory body, in sucha manner that said delayed sampled sequence of frequency more slowlyapproaches a time curve of the resonant frequency following on a changeof the resonant frequency, for example, a jump-like change of theresonant frequency, than the sampled sequence of frequency.

According to a third embodiment of the method of the invention, suchfurther comprises a step of producing a sampling sequence of surfacetemperature, namely a sequence of digital temperature values ascertainedat different points in time based on the at least one temperaturemeasurement signal, in order to approximate a time curve of thetemperature on the second surface of the vibratory body.

According to a fourth embodiment of the method of the invention, suchfurther comprises a step of producing an estimated sequence oftemperature of the vibratory body, namely a sequence of digitaltemperature values ascertained at different points in time based on theat least one temperature measurement signal, in order to approximate atime curve of the temperature of the vibratory body, in such a mannerthat said estimated sequence of temperature of the vibratory body morequickly approaches a time curve of the temperature of the vibratory bodyfollowing on a change of the temperature on the second surface of thevibratory body, for example, a jump-like change and/or a changeresulting from a change of the fluid temperature, than the temperaturemeasurement signal.

According to a fifth embodiment of the method of the invention, suchfurther comprises a step of applying a strain sensor mechanicallycoupled with the vibratory body via its second surface for producing adeformation measurement signal representing a time curve of adeformation of the vibratory body, namely a deformation of the vibratorybody dependent on the temperature of the vibratory body and/or a forceacting on such. Developing this embodiment of the invention further, itis, furthermore, provided to produce, and to apply for producing themeasured value of density, a sampling sequence of deformation, namely asequence of digital deformation measurement values ascertained atdifferent points in time based on the at least one deformationmeasurement signal, in order to approximate a time curve of thedeformation of the vibratory body.

According to a sixth embodiment of the invention, it is, furthermore,provided that the vibratory body is an oscillatably held measuring tubehaving a lumen surrounded by a tube, or pipe, wall, especially a wall ofmetal. Developing this embodiment of the invention further, themeasuring tube is, furthermore, adapted to be immersed in fluid in sucha manner that the first surface of the vibratory body contacting thefluid is formed by an outer surface of the tube, or pipe, wall and thenon fluid contacting, second surface of the vibratory body by an innersurface of the tube, or pipe, wall facing the lumen. Alternativelythereto, the measuring tube can, however, also be adapted to carryfluid, for example, flowing fluid, wherein the first surface of thevibratory body contacting the fluid is formed by an inner surface of thetube, or pipe, wall facing the lumen and the non fluid contacting,second surface of the vibratory body by an outer surface of the tube, orpipe, wall.

According to a seventh embodiment of the invention, the vibratory bodyis adapted to convey fluid, respectively to have fluid flowing throughit.

According to an eighth embodiment of the invention, the vibratory bodyis adapted to be immersed in fluid, or to be flowed on by fluid.Developing this embodiment of the invention further, it is, furthermore,provided that the vibratory body has an oscillatably held membrane, andthat the first surface of the vibratory body contacting the fluid isformed by means of a first membrane surface and the non fluidcontacting, second surface by a second membrane surface lying oppositethe first membrane surface. In the case of this embodiment of theinvention, the vibratory body can further have, for example, also apaddle affixed on the first membrane surface, thus a paddle protrudinginto the fluid.

A basic idea of the invention is at least partially to compensatepreviously unrecognized, dynamic measuring errors inherent in measuringsystems of the type being discussed, errors such as can occur during atransition time period transient as regards the temperature of thevibratory body and resulting, for instance, from a fluid change and/orfrom a significant change of the fluid temperature. Such compensation isachieved by corresponding correcting of the measured resonant frequencyand/or the temperature measured on the vibratory body, namely bysubsequent conforming of the time curve of the measured resonantfrequency to the time curve of the measured temperature regularlytrailing relative to the measured resonant frequency, or by subsequentconforming of the time curve of the measured temperature to the timecurve of the measured resonant frequency leading relative to themeasured temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as other advantageous embodiments thereof will nowbe explained in greater detail based on examples of embodimentspresented in the figures of the drawing. Equal parts are provided in allfigures with equal reference characters; when perspicuity requires or itotherwise′ sensible, already mentioned reference characters are omittedin subsequent figures. Other advantageous embodiments or furtherdevelopments, especially also combinations of first only individuallyexplained aspects of the invention, will become evident, furthermore,from the figures of the drawing, as well as also from the dependentclaims per se. The figures of the drawing show as follows:

FIGS. 1 and 2 shows a measuring system (here embodied in the form of acompact measuring device) of industrial measuring- and automationtechnology for measuring density of a fluid flowing in a pipeline;

FIG. 3 shows schematically in the manner of a block diagram, a measuringsystem of FIG. 1 having an electronics and a measuring transducerconnected thereto;

FIG. 4 shows schematically, a sketch of the principles of a measuringarrangement suitable for a measuring system according to FIG. 1, or ameasuring transducer according to FIGS. 2 and 3, comprising a vibratorybody, an oscillation exciter, an oscillation sensor and a temperaturesensor;

FIG. 5 shows in exploded view, a variant of a measuring transducer,especially a measuring transducer suitable for a measuring systemaccording to FIG. 1, respectively FIG. 2, having a vibratory body formedby means of a measuring tube;

FIG. 6a shows time curves ascertained by means of a measuring transduceraccording to FIGS. 2, 3, and/or 4 of an actual density to be measured, ameasured resonant frequency of the vibratory body, an temperaturemeasured on the vibratory body, as well as a measured density derivedtherefrom in conventional manner; and

FIGS. 6b and 6c show time curves ascertained by means of a measuringtransducer according to FIGS. 2, 3, and/or 4 of an actual density to bemeasured, a corrected resonant frequency of the vibratory body,respectively a corrected temperature of the vibratory body as well as ameasured density derived therefrom.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

FIGS. 1, 2 and 3 show measuring systems schematically, by way ofexample, especially measuring systems suitable for application inindustrial measuring- and automation technology. Such measuring systemsserve to measure a density ρ of a flowable fluid FL, for example, thus aliquid, or a gas, guidable in a line, such as, for instance, a pipelineor a flume, or containable in a container, such as, for instance, atank, and thus to produce sequential measured values X_(ρ) representingsaid density as a function of time. The measuring system is implementedhere, in each case, as an in-line measuring device, namely a measuringsystem insertable into the course of a pipeline (not shown). Themeasuring system can accordingly be, for example, a Coriolis, massflow/density measuring device for measuring, supplementally to densityρ, also mass flow rate m of flowing fluids being measured and/or adensity/viscosity measuring device for measuring, supplementally todensity, also viscosity η of flowing fluids.

For registering the density, the measuring system comprises a measuringtransducer MT—here a measuring transducer insertable into the course ofa pipeline (not shown). During operation, the fluid to be measured flowsthrough the measuring transducer. Measuring transducer MT includes anoscillatably held, vibratory body 10, especially one of metal and—asdirectly evident from the combination of FIGS. 1, 2 and 3—iselectrically connected to a measuring electronics ME accommodated in anelectronics housing 200 and lastly delivering the density, measuredvalues X. The vibratory body 10 has a plurality of eigenfrequencies, ofwhich each is determined decisively by the material, respectivelymodulus of elasticity, as well as by the mechanical construction,respectively the actual installed situation, of the vibratory body.

The vibratory body 10 is, as schematically presented in FIG. 4, adapted,during operation, to be contacted, at least on a first surface 10+, bythe fluid FL to be measured and simultaneously to be excited actively toexecute mechanical oscillations, and, indeed, in such a manner that thevibratory body contacted by fluid 10 executes, at least partially,resonant oscillations o_(r), namely, mechanical oscillations with aresonant frequency f_(r), which—besides depending on one of theeigenfrequencies—also depend on the density ρ of the fluid contactingthe first surface of the vibratory body and can consequently serve as ameasure for said density. Moreover, the resonant frequency f_(r) is, asis known, additionally determined also by a fluid temperature Θ_(FL),namely a temperature of the fluid contacting the first surface, sincethe eigenfrequencies of the vibratory body, not least of all because ofa temperature dependence of a modulus of elasticity of the vibratorybody, as well as also a temperature dependent, volume expansion, isinfluenced decisively also by a temperature Θ₁₀, especially an averagetemperature, of the vibratory body, namely a temperature of thevibratory body dependent on the fluid temperature Θ_(FL).

Based on these mechanical oscillations of the vibratory body 10, themeasuring transducer generates, furthermore: At least one oscillationmeasurement signal s_(sens1) dependent on density, namely having atleast one signal component with a signal frequency corresponding to theresonant frequency f_(r) and representing, consequently, vibrations ofthe vibratory body 10; as well as at least one temperature measurementsignal θ_(sens) serving for compensating for the influence oftemperature of the vibratory body 410 on the resonant frequency f_(r),consequently on the oscillation measurement signal s_(sens1). The signalθ_(sens) corresponds to, in any event, at least approximatelyrepresents, a time curve of the temperature 410 of the vibratory body.

The measuring electronics ME includes, as schematically presented inFIG. 3, consequently, furthermore, a driver-circuit Exc serving foractivating the measuring transducer as well as a measuring- andevaluating-circuit μC serving for processing the at least oneoscillation measurement signal s_(sens1) of the measuring transducer MTand formed, for example, by means of at least one microprocessor and/orby means of a digital signal processor (DSP). Measuring- andevaluating-circuit μC applies the at least one oscillation measurementsignal s_(sens1) to produce the density, measured values, which can, forexample, also be in the form of digital values.

The density-measured values generated by means of the electronics MEcan, for example, be displayed on-site. For visualizing, on-site,measured values produced internally in the measuring device and/or, ingiven cases, measuring device internally generated, system statusreports, such as, for instance, an error report or an alarm, themeasuring device can, as also indicated in FIG. 3, have, for example, adisplay- and servicing element HMI, which is in communication with theelectronics and is, in given cases, also portable. Examples of thedisplay- and servicing element HMI include, for instance, an LCD-, OLED-or TFT display placed in the electronics housing behind a windowprovided correspondingly therein, as well as a corresponding inputkeypad and/or a touch screen. In advantageous manner, the, for example,also remotely parameterable, electronics can, furthermore, be sodesigned that it can, during operation of the measuring device, exchangewith a superordinated electronic data processing system, for example, aprogrammable logic controller (PLC), a personal computer and/or a workstation, via a data transmission system, for example, a fieldbus systemand/or wirelessly per radio, measuring—and/or other operating data, suchas, for instance, current measuring- and/or system diagnosis values ortuning values serving for control of the measuring device. Furthermore,the electronics ME can be so designed that it can be fed by an externalenergy supply, for example, also via the aforementioned fieldbus system.For the case, in which the measuring device is to be coupled to afieldbus- or other communication system, the electronics ME, forexample, also an on-site and/or via communication system(re-)programmable, electronics ME, can have, additionally, acorresponding communication-interface for data communication, e.g. forsending measuring- and/or operating data, thus, the measured valuesrepresenting the at least one measured variable, to the alreadymentioned, programmable logic controller or to a superordinated processcontrol system and/or for receiving settings data for the measuringdevice. Particularly for the case, in which the measuring device is tobe coupled to a fieldbus- or other communication system, the electronicsME includes, consequently, furthermore, a communication interface COMembodied for data communication according to one of the relevantindustry standards. Moreover, the electronics ME can have, for example,an internal energy supply circuit ESC, which, during operation, is fedvia the aforementioned fieldbus system by an external energy supplyprovided in the aforementioned data processing system. In such case, theelectronics can, furthermore, be so embodied e.g. that it iselectrically connectable with the external electronic data processingsystem by means of a two-wire connection 2L configured, for example, asa 4-20 mA-current loop, and thereby be supplied with electrical energyas well as be able to transmit measured values to the data processingsystem; the measuring device can, however, for example, also be embodiedas a so-called four-conductor-measuring device, in the case of which theinternal energy supply circuit ESC of the electronics ME is connected bymeans of a first pair of lines with an external energy supply and theinternal communication circuit COM of the electronics ME by means of asecond pair lines with an external data processing circuit or anexternal data transmission system.

The measuring electronics ME is, furthermore, in the example of anembodiment shown here, accommodated in a corresponding electronicshousing 200, especially one impact- and/or also explosion resistantlyand/or hermetically sealedly formed and/or modularly built up.Electronics housing 200 can, for example, be arranged removed from themeasuring transducer, or, as shown in FIG. 1, be affixed directly on themeasuring transducer MT, for example, externally on the transducerhousing 100, in order to form a single, compact device. In the case ofthe example of an embodiment shown here, there is, consequently, placedon the transducer housing 100, furthermore, a necklike transition pieceserving for holding the electronics housing 200. Arranged within thetransition piece can be, furthermore, a hermetically and/or pressureresistantly sealed duct, for example, sealed by means of glass and/orplastic potting compound, for electrical connecting lines betweenelectrical components of the measuring transducer MT, here, for example,thus the oscillation exciter, or the oscillation sensor, and theelectronics ME.

For active exciting of vibrations of the vibratory body, especially alsothe resonant oscillations required for measuring density, the measuringtransducer comprises, furthermore, at least one electro-mechanicaloscillation exciter 41 in actuating connection with the vibratory body—, for example, an electrodynamic oscillation exciter 41, namely formedby means of an armature extending movably in a solenoidal coil. Saidoscillation exciter 41 is, as schematically presented in FIG. 2,respectively FIG. 4, respectively directly evident from theircombination, arranged at a second surface 10# facing away from the firstsurface 10+ of the vibratory body 10—namely that surface not contactedduring operation by the fluid to be measured—and serves, in such case,especially, to convert an electrical excitation power P_(exc) fed fromthe driver circuit Exc of the electronics ME by means of at least oneelectrical driver signal s_(drv) into, e.g. pulsating or harmonic,namely essentially sinusoidal, exciter forces F_(exc), which actcorrespondingly on the vibratory body 10 and, thus, actively excite thedesired resonant oscillations. For example, the at least one driversignal s_(drv) can have simultaneously also a plurality of sinusoidalsignal components with signal frequencies differing from one another, ofwhich one —, for instance, one at least at times dominating as regardssignal power—signal component has a signal frequency corresponding tothe resonant frequency f_(r) required for measuring density. The exciterforces F_(exc) generated by converting electrical excitation powerP_(exc) fed into the oscillation exciter can, in such case, in mannerknown, per se, to those skilled in the art, be correspondingly tuned bymeans of the driver circuit Exc provided in the electronics ME, forinstance, by means of electrical current- and/or voltage-controllersimplemented in the driver circuit, controlling an amplitude (electricalcurrent level) of an electrical current of the driver signal and/or anamplitude (voltage level) of a voltage of the driver signal as regardstheir magnitude, and e.g. by means of a phase control loop (PLL—phaselocked loop) likewise provided in the driver circuit Exc, as regardstheir instantaneous frequency or, in the case of multifrequencyexcitation, as regards their instantaneous frequencies, compare, forthis, for example, also U.S. Pat. No. 4,801,897 or U.S. Pat. No.6,311,136. The construction and application of the aforementioned phasecontrol loop for the active exciting of vibratory bodies of the typebeing discussed to an instantaneous resonant frequency is described atlength e.g. in U.S. Pat. No. 4,801,897. Of course, also other drivercircuits known, per se, to those skilled in the art to be suitable fortuning the exciter energy E_(exc) can be used, for example, also thoseset forth in the initially mentioned state of the art, for instance, theinitially mentioned U.S. Pat. No. 4,777,833, U.S. Pat. No. 4,801,897,U.S. Pat. No. 4,879,911, U.S. Pat. No. 5,009,109, U.S. Pat. No.5,024,104, U.S. Pat. No. 5,050,439, U.S. Pat. No. 5,804,741, U.S. Pat.No. 5,869,770, U.S. Pat. No. 6,073,495, or U.S. Pat. No. 6,311,136.Furthermore, as regards an application of such driver circuits,reference is made to the electronics provided with measurementtransmitters of the sensor series “PROMASS 83”, as available from theassignee, for example, in connection with measuring transducers servingalso for measuring density in the sensor series “PROMASS E”, “PROMASSF”, “PROMASS H”, “PROMASS I”, “PROMASS P”, “PROMASS S”, or “PROMASS X”.Their driver circuit is, for example, in each case, so executed thatresonant oscillations are controlled to a largely constant amplitude,consequently also independent of the density ρ, or also the viscosity η,of the respective fluid to be measured.

For registering vibrations of the vibratory body 10, not least of allalso the resonant oscillations o_(r) actively excited by means of the atleast one oscillation exciter 41, as well as for transducing saidregistered vibrations into the at least one oscillation measurementsignal s_(sens1), the measuring transducer MT includes, furthermore, atleast a first oscillation sensor 51, for example, an electrodynamicsensor, —here one spaced from the at least one oscillation exciter41—arranged on the second surface 10# of the vibratory body 10.Oscillation sensor 51 lastly delivers the oscillation measurement signals_(sens1) representing vibrations of the vibratory body, for example, inthe form of an electrical (alternating-)voltage corresponding to theoscillations with an amplitude (voltage level) dependent on aninstantaneous amplitude of the oscillations of the vibratory body and afrequency corresponding to that of the resonant frequency f_(r).Moreover, the measuring transducer includes additionally at least onetemperature sensor 61 thermally coupled with the vibratory body via itssecond surface 10#, for example, adhered thereon, for producing thementioned temperature measurement signal θ_(sens).

As already mentioned and also schematically presented in FIG. 2,respectively 5, vibratory body 10 can be formed, for example, by meansof a measuring tube flowed through during operation by the fluid FL. Themeasuring tube is accommodated in a measuring transducer housing 100 andheld oscillatably therein. The measuring tube has a lumen surrounded bya tube wall, especially one of metal, and extends with a desiredoscillatory length between an inlet-side, first measuring tube end 10′and an outlet-side, second measuring tube end 10″. The measuring systemcan accordingly be embodied, for example, also as a Coriolis, massflow/density measuring device measuring, supplementally to density, alsoa mass flow of the fluid flowing FL and/or as a viscosity/densitymeasuring device measuring supplementally to density also a viscosity ofthe fluid. The measuring tube 11, consequently the vibratory body formedtherewith, is adapted, in such case, to be flowed through by the fluidto be measured, consequently to carry the volume portion of the fluid tobe measured, after it has been allowed to flow into the lumen, whereinthe first surface of the vibratory body contacting the fluid is formedby an inner surface of the tube wall facing the lumen and contacted bythe fluid and the non fluid contacting, second surface of the vibratorybody is formed by an outer surface of the tube wall. The resonantoscillations serving for measuring density can, in such case, forexample, be such that the measuring tube serving as vibratory body iscaused to vibrate over its entire desired oscillatory length, forexample, in a bending oscillation mode, in which the at least onemeasuring tube deflects about an oscillation axis imaginarily connectingthe two measuring tube ends 10′, 10″ with one another and extendingessentially parallel to an imaginary longitudinal axis L of themeasuring transducer, in the manner of a unilaterally clampedcantilever, and, in such case, is deformed oscillatingly, repeatedly,elastically about a static resting position. The desired oscillatorylength corresponds, in such case, virtually to a length of a middle, oralso centroidal, axis (connecting line through the centers of gravity ofall cross sectional areas of the measuring tube) extending within thelumen—in the case of a curved measuring tube, thus, a straightenedlength of the measuring tube. The measuring transducer resembles, in itsmechanical construction, as well as also its principle of action, themeasuring transducers proposed in U.S. Pat. No. 7,360,451 or U.S. Pat.No. 6,666,098, or also those available from the assignee under the marks“PROMASS H”, “PROMASS P” or “PROMASS S” for measuring both density aswell as also mass flow of flowing fluids. For implementing theinvention, however, also other measuring transducers with vibratorybodies can serve, in the case of measuring transducers with a measuringtube as vibratory body, thus, also such with straight and/or greaterthan one measuring tube, for example, thus four or, as shown in FIG. 5,two measuring tubes or also such comparable in the initially mentionedUS-A 2010/0236338, US-A 2010/0242623, US-A 2010/0242624, U.S. Pat. No.5,602,345, U.S. Pat. No. 5,731,527, U.S. Pat. No. 5,796,011, U.S. Pat.No. 6,006,609, U.S. Pat. No. 6,513,393, U.S. Pat. No. 6,840,109, U.S.Pat. No. 6,920,798 or U.S. Pat. No. 7,017,424, or, for example, also themeasuring transducers available from the assignee under the marks“PROMASS I”, “PROMASS M”, “PROMASS E”, “PROMASS F”, or “PROMASS X” formeasuring mass flow as well as also density of flowing fluids. Inaccordance therewith, the measuring transducer can, for example, also bea single straight measuring tube or have at least two measuring tubes,each serving as a vibratory body, for example, mechanically coupled withone another by means of an inlet-side flow divider and an outlet-sideflow divider, in given cases, supplementally also by means of in- andoutlet-side coupling elements, and/or equally constructed measuringtubes and/or curved ones and/or extending parallel to one another, forconveying the fluid to be measured. During operation, the at least twomeasuring tubes vibrate opposite-equally to one another, at least attimes, for producing the oscillation measurement signals, for instance,with equal frequency at a shared oscillation frequency. Particularly forthe case, in which the vibratory body is formed by means of a straightmeasuring tube, the resonant oscillations o_(r) can be, for example,also in the form of torsional oscillations o_(r) radial oscillationsabout an oscillation axis parallel to, in given cases, even coincidentwith, the mentioned longitudinal axis of the measuring transducer.

For the typical case for such a measuring transducer with measuring tubeserving as vibratory body, when said measuring transducer MT is to beassembled releasably with the process line, for example, a process linein the form of a metal pipeline, there are provided, as indicated inFIG. 1, 2, or 5, or as directly evident from their combination, on theinlet side (100+) of the measuring transducer, a first connecting flange13 for connection to a line segment of the process line supplying fluidto the measuring transducer and, on the outlet side (100#), a secondconnecting flange 14 for a line segment of the process line removingfluid from the measuring transducer. The connecting flanges 13, 14 can,in such case, as quite usual in the case of measuring transducers of thedescribed type, also be integrated terminally in the measuringtransducer housing 100, consequently form an inlet-side measuringtransducer end 100+, respectively an outlet-side measuring transducerend 100#. In the case of its application in a Coriolis, mass flowmeasuring device, the measuring transducer, according to an additionalembodiment of the invention, includes, furthermore, a second oscillationsensor 52 spaced from the first oscillation sensor 51 in the flowdirection, wherein the first oscillation sensor is placed, for example,on the inlet side of the measuring tube serving as vibratory body, whilethe second oscillation sensor is arranged downstream from the firstoscillation sensor, on the outlet side of the measuring tube.

Instead of a measuring transducer with a measuring tube serving asvibratory body, or a measuring system formed therewith, for example,thus implementable as a Coriolis, mass flow-/density measuring device,the density can, however, also be measured by means of some otherelectro-mechanical oscillatory system serving as vibratory body andbringable in contact with the fluid to be measured. Serving forimplementing the invention can be, for example, also such densitymeasuring systems, in the case of which the vibratory body is adapted,for the purpose of registering the density, to be at least partiallyimmersed in, or flowed on by, the fluid to be measured. In accordancetherewith, the measuring system can, for example, thus also be aso-called fill level limit switch with integrated density measurementhaving at least one, for example, paddle shaped appendage and/orinternally hollow, oscillatory rod, for example, thus according to theinitially mentioned U.S. Pat. No. 6,845,663, so that the vibratory bodycan have an oscillatably held membrane, in such a manner that the firstsurface of the vibratory body contacting the fluid is formed by means ofa first membrane surface and the non fluid contacting, second surface bya second membrane surface lying opposite the first membrane surface, orthe vibratory body can further have a paddle affixed on the firstmembrane surface, in order that the paddle can protrude into the fluid.Furthermore, the measuring system can be, for example, a Coriolis, massflow-/density measuring device with a vibratory body in the form of ahollow body, which can be inserted through a wall of a pipeline, suchas, for instance, according to the initially mentioned EP-A 564 682,such as in the form of an at least unilaterally sealed, internallyhollow cylinder.

The at least one oscillation measurement signal s_(sens1) generated bythe measuring transducer as well as also the temperature measurementsignal θ_(sens) are, as is schematically presented FIGS. 2 and 3, or asdirectly evident from their combination, fed to the electronics ME, inorder there, first of all, to be preprocessed, especially preamplified,filtered and digitized by means of an input circuit FI of theelectronics connected in front of the therein provided, actualmeasuring- and evaluating circuit μC. After this preprocessing,evaluation follows, namely, the signal is at least converted into the atleast one measured value of density X_(ρ), or other, later, measuredvalues of density; this is done, in given cases, also taking intoconsideration electrical excitation power fed into the exciter mechanismby means of the at least one driver signal, there to be converted intoexcitation force. Particularly for the purpose of generating themeasured value of density X_(ρ), the measuring- and evaluating circuitμC ascertains, based on the oscillation measurement signal s_(sens1),recurringly a measured value X_(f) of frequency serving as measure for aresonant frequency, consequently representing said resonant frequencyand forming a basis for determining the current measured value X_(ρ) ofdensity, as well as, based on the temperature measurement signalθ_(sens), at times, also a measured value X_(Θ) of temperature, whichserves as a measure for the one temperature of the vibratory body thatdetermines the eigenfrequency forming a basis for determining thecurrent measured value of density X_(ρ), consequently representing saidtemperature of the vibratory body. With the measured value X_(f) offrequency and the measured value X_(Θ) of temperature, the measuredvalue X_(ρ) of density can be ascertained in manner, per se, familiar tothose skilled in the art, for instance, based on the known approximationformula

${{{\left. X_{\rho} \right.\sim K} \cdot \frac{1 + X_{\vartheta}}{X_{f}^{2}}} + \cdots}\mspace{14mu},$

thus by dividing the measured value X_(Θ) of temperature by a squaredmeasured value X_(f) of frequency.

In the case of application in a Coriolis, mass flow-measuring device,the electronics ME serves, furthermore, also, with application of theoscillation measurement signals generated by the measuring transducer,namely based on a phase difference detected between the oscillationmeasurement signals s_(sens1), s_(sesn2) of the first and secondoscillation sensors 51, 52, as caused by Coriolis forces in the flowingfluid, recurringly to ascertain a mass flow measured value X_(m), whichrepresents a mass flow rate, m, to be measured for fluid guided throughthe measuring transducer. Alternatively thereto or in supplementationthereof, the measuring- and evaluating circuit can, as quite usual inthe case of measuring systems formed by means of a vibratory body formeasuring density, in given cases, also be applied, based on the fed-inelectrical excitation power P_(exc) as well as the at least oneoscillation measurement signal s_(sens1), to ascertain a viscositymeasured value X_(η) representing a viscosity η of the fluid; compare,for this, also the initially mentioned U.S. Pat. No. 7,284,449, U.S.Pat. No. 7,017,424, U.S. Pat. No. 6,910,366, U.S. Pat. No. 6,840,109,U.S. Pat. No. 5,576,500 or U.S. Pat. No. 6,651,513.

The program code for such evaluating programs serving for generatingmeasured values, not least of all also the density, measured values, orcontrol programs serving for operating the measuring transducer can bestored e.g. persistently in a non-volatile data memory EEPROM of theelectronics and, upon start up of the electronics, loaded into avolatile data memory RAM, e.g. one integrated into the processor.Equally, measured values generated by means of the electronics ME duringoperation can be loaded into such a (in given cases, also the same)volatile, or into such a non-volatile, data memory and correspondinglyheld for later, further processing.

As already mentioned, methods for ascertaining the density of fluidsbased on resonant oscillations of a vibratory body can, at times,exhibit considerable measuring inaccuracies, this, especially being thecase during a transition time period directly following on a change ofthe fluid FL, during which time period the measuring system transfersfrom a steady state occupied before the fluid change into a changedstate determined by the new fluid. Further investigations on measuringsystems of the type being discussed have led to the result that suchmeasuring inaccuracies can be explained partially by the fact that sucha fluid change can, on the one hand, regularly also be accompanied witha significant change of the fluid temperature Θ_(FL) effective for themeasuring, on the other hand, however, exactly this change of the fluidtemperature, or the effect thereof on the entire measuring system, hasso far been looked upon as negligible, or completely overlooked, in anyevent, however, not sufficiently taken into consideration, in thefunctions f_(r) ²=f(1/φ, or f_(r) ²=f(Θ₁₀), Θ_(sens)˜Θ₁₀ holdingactually only for steady state conditions. The vibratory body 10 has,namely, both a certain heat capacity, C₁₀, as well as also a specificthermal conductivity λ₁₀—amounting usually to greater than 5 W K⁻¹m⁻¹—and accordingly a thereon dependent, effective thermal conductance,Λ₁₀, consequently a certain thermal inertia, for heat transfer from thefirst surface 10+ of the vibratory body, naturally having the fluidtemperature, to its second surface 10#. Thus, the temperature of thevibratory body Θ₁₀ changes not only directly after a change of the fluidtemperature initiated, for example, by a fluid change, but, also as aresult of its thermal inertia dependent on the heat capacity, C₁₀ andthe thermal conductance, Λ₁₀, also over a certain amount of time muchlonger than a measuring cycle time required for ascertaining a measuredvalue of density X_(ρ). Associated therewith, however, also thetemperature Θ₂→θ_(sens) actually registered by the temperature sensor 61on the second surface 10# of the vibratory body 10 deviates during thetransition time period always from the temperature of the vibratory bodyΘ₁₀ actually effective for the oscillation characteristics, and, indeed,in an amount changing as a function of time. Alone already because ofthis, thus the temperature measurement signal θ_(sens) can follow,however time delayed, a change of the temperature of the vibratory bodyΘ₁₀ from a beginning first temperature value Θ_(10,t1) to a secondtemperature value Θ_(10,t2) for example, thus a change resulting from achange of the temperature of the fluid contacting the vibratory body onits first surface 10+ and/or a fluid change); consequently, thetemperature measurement signal θ_(sens) corresponds to said secondtemperature value, only time delayed, however, without that this, or atherefrom resulting dynamic measuring error in the case of ascertainingthe density, has so far been correspondingly taken into consideration.For example, thus the temperature Θ₂→θ_(sens) on the second surface 10#of the vibratory body 10, thus the temperature signal θ_(sens, can),during a transition time period of the aforementioned type, exhibit atime curve, which approximately corresponds to that shown in FIG. 6 a.

As another cause for measuring inaccuracies of the aforementioned type,there is also an inherent thermal inertia of the temperature sensor,which yet further increases the dynamic measuring errors of conventionalmeasuring systems of the type being discussed.

As a result of such temperature Θ₁₀ of the vibratory body continuallychanging during the transition time period, yet so far, however, notcorrectly, or not at all, considered in the ascertaining, there has tochange correspondingly also the oscillation characteristics of thevibratory body 10, so that, thus, a change of the resonant frequencyf_(r) associated with the fluid change is, as a result, not—aspreviously—alone to be attributed to the corresponding change of thedensity, but, instead, additionally also to a thermally related changeof the eigenfrequency co-determining the resonant frequency f_(r) of thevibratory body 10. Furthermore, this means that, during the transitiontime period for the temperature Θ₁₀ of the vibratory body, the resonantfrequency ascertained based on the at least one oscillatory signals_(sens1) in the form of corresponding frequency measured valuesX_(f)—for which, in FIG. 6a , by way of example, a corresponding timecurve containing said transition time period is shown—can, indeed,correspond very exactly to the actual resonant frequency f_(r),nevertheless, however, therefrom derived density, measured valuesX′_(ρ)—of which likewise a corresponding time curve is shown in FIG. 6acan, in the meantime, deviate in considerable measure from theinstantaneous density ρ, because the temperature Θ₂→Θ_(sens) on thesecond surface 10# of the vibratory body 10 utilized for itsascertaining, consequently the therefrom, in each case, derived,measured values X_(Θ) of temperature only approximately reflect theactually required temperature of the vibratory body Θ₁₀, or a time curvetherefrom, as is directly evident from FIG. 6a . As a result of this,the density X′_(ρ) ascertained in conventional manner for theabove-described time curve of measured resonant frequency (f_(R)→X_(f))and temperature measured (Θ₂→X_(Θ)), namely without corresponding takinginto consideration of the dynamic behavior of the temperature measuringchain, can approximately correspond to the curve shown in FIG. 6a ,wherein, clearly recognizably, not only a density erroneous inconsiderable measure is ascertained, but, also, first of all, namelydirectly after completed fluid change, regretfully even an increasingdensity is suggested, although it has really decreased from what itwas—for instance, as a result of using a fluid, which is only warmer,but otherwise essentially the same.

Taking this into consideration, the measuring system of the inventionis, consequently, furthermore, adapted, and the method implementedtherewith for measuring the density, furthermore, so embodied, that, inproducing the measured value of density X_(ρ), based on the at least oneoscillation measurement signal s_(sens1) as well as the at least onetemperature measurement signal θ_(sens), during a change of thetemperature Θ₁₀ of the vibratory body—for example, thus a temperaturechange resulting from a change of the fluid temperature Θ_(FL), or thetemperature of the vibratory body 10 on its first surface 10+, upon areplacement of a fluid supplied earlier to the vibratory body with adifferently characterized fluid FL—a discrepancy Err′_(Θ)=θ_(sens)−Θ₁₀occurring during the producing of said measured value of density betweenthe time curve of the temperature of the vibratory body and thetemperature measurement signal is taken into consideration. In thesimplest case, the taking into consideration of the discrepancy can bethat its occurrence is detected and signaled, for example, in the formof a report indicated on-site, and/or is documented by storing in thedata memory, in given cases, accompanied by a time stamp.

In an additional embodiment of the invention, it is, furthermore,provided that the discrepancy, which usually gets smaller with time, andis, consequently, time dependent, is taken into consideration already inthe ascertaining of the measured value X_(f) of frequency and/or themeasured value X_(Θ) of temperature, for example, by calculation bymeans of the measuring- and evaluating-circuit μC and/or by suitablyconditioning the temperature measurement signal θ_(sens) by means of asignal filter correspondingly adapted as regards its transfer behavior,and, indeed, in such a manner that the thermal inertia of the vibratorybody and/or of the temperature sensor, consequently said discrepancy, isat least partially compensated in the case of ascertaining the measuredvalue of density.

Fundamentally, there are essentially thus at least three approachescomputationally, by suitable signal processing of the at least oneoscillation measurement signal as well as the at least one temperaturemeasurement signal, to compensate the aforementioned discrepancyexisting between the time curve of the temperature of the vibratory bodyand the temperature measurement signal and resulting from the thermalinertia of the measuring chain comprising the vibratory body as well asthe temperature sensor contacting such and serving for ascertaining thetemperature of the vibratory body. These approaches include acorresponding delaying (namely making movement of the resonant frequencytoward a steady end value slower than it would otherwise be) of a timecurve of the resonant frequency of the vibratory body (FIG. 6b )ascertainable based on signal processing of the oscillation measurementsignal, or by a corresponding accelerating (namely, making temperaturemove faster toward a steady end value than the temperature registered onthe second surface 10# of the vibratory body 10) of a time curve of thetemperature on the second surface of the vibratory body 10 ascertainablebased on signal processing of the temperature measurement signal (FIG.6c ) or by a corresponding combination of said signal processingdelaying of the time curve of the resonant frequency and said signalprocessing accelerating of the time curve of the temperature on thesecond surface of the vibratory body.

In an additional embodiment of the invention, consequently, themeasuring- and evaluating-circuit μC, first of all, produces a sampledsequence f_(D1) of frequency, namely a sequence of digital frequencyvalues X_(f1) ascertained at different points in time, for example atequidistant points in time based on the at least one oscillationmeasurement signal s_(sens1) and approximating, namely at leastapproximately corresponding to (f_(D1)˜f_(r)), a time curve of theresonant frequency f_(r) of the vibratory body. Furthermore, it isprovided according to a variant of the invention further developing thisembodiment that the sampled sequence f_(D1) of frequency is applied forproducing a delayed sampled sequence f_(D2) of frequency, namely asequence of digital frequency values X_(f2) ascertained at differentpoints in time, for example, at equidistant points in timet_(n)=n·T_(s), consequently with a clocking rate, especially a constant,clocking rate, f_(s)=1/(t_(n+1)−t_(n))=1/T_(s), based on the sampledsequence f_(D1) of frequency, and—as schematically presented in FIG. 6b—approximating the time curve of the resonant frequency f_(r) of thevibratory body in such a manner that said delayed sampled sequencef_(D2) of frequency approaches an actual time curve f_(r)(t) of theresonant frequency f_(r) following on a change of the resonantfrequency, for example, a ramp shaped or even, as shown in FIG. 6b , ajump shaped change of the resonant frequency, more slowly than thesampled sequence of frequency f_(D) (corresponding to the time curvef_(r)→X_(f) in FIG. 6a ). An, in each case, currently (namely,ascertained at the point in time t_(n)) ascertained frequency valueX_(f2)[n] of the delayed sampled sequence of frequency f_(D2) serveshere, in each case, then also as current measured value of frequencyX_(f2)[n]→X_(f)[n]. As a result of this, thus a currently ascertained,digital measured value X_(f)[n] of frequency during a transition timeperiod transient at least as regards the temperature of the vibratorybody deviates, for example, as a result of a change of fluid or a changeof the fluid temperature, always from the actually, or instantaneously,registered resonant frequency f_(r) at the point in time t_(n) by acertain difference amount Err_(f)=X_(f)[n]−f_(r) becoming smaller as theend of the transition time period is approached, in order ultimately,after reaching again a new steady state of the measuring systemdetermined by the instantaneous fluid temperature and the instantaneousdensity, to correspond again exactly to the actual resonant frequencyf_(r).

Accordingly serving for generating the delayed sampled sequence f_(D2)of frequency, consequently for generating the therefrom derivedfrequency measured values X_(f)[n], can be a digital filter embodied,for example, as an HR filter (infinite response filter) or also an FIRfilter (finite response filter), which has a transfer functionG*(z)=z(g[n]) corresponding to a lowpass filter of first or even higherorder. In the case of application of a FIR filter (finite responsefilter) as digital filter, the transfer function is defined, as isknown, by the simple calculational formula

${{G^{*}(z)} = {{Z\left( {g\lbrack n\rbrack} \right)} = {{\sum\limits_{k = 0}^{N}{w_{k} \cdot z^{- k}}} = {{\sum\limits_{k = 0}^{N}{w_{k} \cdot ^{{- j}\; \omega \; T_{s}}}} = {\sum\limits_{k = 0}^{N}{w_{k} \cdot ^{{- j}\; \omega \; {({t_{n + 1} - t_{n}})}}}}}}}},$

wherein, in the case of a digital filter formed as an interpolatorcorresponding to a pure lowpass filter, all of the filter coefficientsw_(k) have positive sign. In accordance therewith, as in the case of anFIR filter, for the purpose of generating the delayed sampled sequencef_(D2) of frequency, consequently for the purpose of ascertaining a, ineach case, current measured value of frequency X_(f)[n], sequentialfrequency values of the sampled sequence f_(D1) of frequency are summedwith weighting according to the calculational recipe

$\left. {X_{f}\lbrack n\rbrack}\leftarrow{X_{f2}\lbrack n\rbrack} \right. = {\sum\limits_{k = 0}^{N}{w_{k} \cdot {X_{f1}\left\lbrack {N - k} \right\rbrack}}}$

representing the aforementioned transfer function, consequently thedigital filter, in the sampling range. The parameters defining thetransfer function, namely the filter coefficients w_(k), as well as afilter length N corresponding to their number, can, in such case,independently of the type of filter (IIR-, or FIR filter), be soselected, for example, that the therewith established transfer functionof the digital filter approaches the dynamic transfer behavior of themeasuring chain formed by means of the vibratory body and thetemperature sensor, namely its thermal inertia, so that thus a timecurve of the delayed sampled sequence f_(D2) of frequency correspondsduring a transient transitional period of the above-described type tothe time curve of the temperature on the second surface of the vibratorybody 10, or to the time curve of the temperature signal representingsuch.

In an additional variant of the invention, not least of all for thepurpose of ascertaining the measured value X_(Θ) of temperature, bymeans of the measuring- and evaluating-circuit μC, an estimated sequenceSEM of temperature of the vibratory body, namely a sequence of digitaltemperature values X_(Θ1) based on the at least one temperaturemeasurement signal ascertained at different points in time, for example,equidistant points in time t_(m)=m·T_(s2), consequently with a clockingrate, especially a constant clocking ratef_(s2)=1/(t_(m+1)−t_(m))=1/T_(s2), is formed to approximate a time curveof the temperature of the vibratory body, in such a manner that saidestimated sequence of temperature of the vibratory body—as schematicallypresented in FIG. 6c , or also evident from a combination of FIGS. 6aand 6c —approaches a time curve of the temperature of the vibratory bodyfollowing on a change of the temperature on the second surface of thevibratory body —, for instance, a jump-like change and/or a changeresulting from a change of the fluid temperature—more quickly than thetemperature measurement signal θ_(sens). A temperature value X_(Θ1)[m]of the estimated sequence Θ_(D1) of temperature of the vibratory body,in each case, currently ascertained, namely ascertained for the point intime t_(m), serves here, in each case, then also as current measuredvalue of temperature X_(Θ1)[m]→X_(Θ)[m]. As a result of this, thus adigital measured value X_(Θ1)[m] of temperature ascertained during atransition time period of the type being discussed deviates currentlyfrom the actual temperature Θ₁₀ of the vibratory body at the point intime t_(m) by a difference amount Err_(Θ)=X_(Θ1)[m]−Θ₁₀, which is atleast smaller than an instantaneous deviation between the temperaturemeasurement signal θ_(sens) delivered at the moment by the temperaturesensor and the instantaneous temperature Θ₁₀ of the vibratory bodycorresponding to the instantaneous discrepancy Err's to be taken intoconsideration—ideally, however, it is as small as possible.

The estimated sequence of temperature of the vibratory body can begenerated, for example, by feeding the temperature measurement signal toan analog signal filter, which has at least one signal transmission pathwith a high pass-characteristic, where, as regards transfer function,the temperature measurement signal is differentiated, such that thefilter is designed as a high pass of first order characterized by onetime constant or a high pass of higher order characterized by a numberof time constants. The signal filter can, in the simplest case, beformed, for example, by means of correspondingly connected resistors,capacitors and/or coils, consequently by means of a filter networkimplemented only with passive electrical components or, however, alsoimplemented as an active signal filter having, namely, additionally alsooperational amplifiers. By suitable trimming of the components definingthe signal filter as regards its transfer function, the signal filtercan, in such case, be so tuned that it at least partially compensatesthe mentioned thermal inertia of the measuring chain, which lastlycauses the dynamic measuring errors during the transient transitionperiod, namely by taking into consideration also time changes of thetemperature measurement signal to deliver a corresponding output signal,which leads the temperature measurement signal, or its time curve,consequently, relative to the actual time curve of the temperature ofthe vibratory body, at least trails less than the temperaturemeasurement signal. The accordingly equally analog, output signal canthereafter be digitized in conventional manner, namely converted intothe—digital—estimated sequence of temperature of the vibratory body. Ofcourse, the estimated sequence of temperature of the vibratory body cane.g., however, also be generated by, first of all, digitizing thetemperature measurement signal, consequently ascertaining a samplingsequence, Θ_(D2), of surface temperature, namely a sequence of digitaltemperature values at different points in time t_(m) is produced basedon the at least one temperature measurement signal, in order toapproximate a time curve of the temperature on the second surface of thevibratory body, and thereafter deriving therefrom the estimated sequenceof temperature of the vibratory body by processing the sampling sequenceof surface temperature by means of a correspondingly adapted digitalfilter, namely a digital filter differentiating the sampling sequence ofsurface temperature, in order to obtain the estimated sequence oftemperature of the vibratory body. The digital filter can be, forexample, a FIR filter with a highpass characteristic, the filtercoefficients w_(k) of which thus have at least two sequential, non-zero,filter coefficients w_(i), w_(i+1) of different sign.

For additionally improving the accuracy of the density, measured valuesascertained by means of the measuring system of the invention, it can beadvantageous, furthermore, supplementally to the temperature of thevibratory body, also to register possible mechanical deformations of thevibratory body, for instance, deformations occurring as a result ofchanging temperature of the vibratory body and/or as a result of forcesacting on the vibratory body, or therefrom resulting mechanical stresseswithin the vibratory body and correspondingly to take such intoconsideration in the case of calculating the density, measured values.Therefore, the measuring system according to an additional embodiment ofthe invention includes a strain sensor (not shown) for producing a timecurve of a deformation of the vibratory body, namely a deformationmeasurement signal representing deformation of the vibratory bodydependent on the temperature of the vibratory body and/or a force actingon such. The strain sensor, embodied, for example, as strain gages, ismechanically coupled with the vibratory body, namely via its secondsurface, and can, for example, be affixed, for example, adhered,directly to the vibratory body in the immediate vicinity of the at leastone temperature sensor. Based on the at least one deformationmeasurement signal, for the purpose of taking the registered strain intoconsideration in generating the at least one measured value of density,this can likewise be digitized, consequently a corresponding samplingsequence of deformation, namely a sequence of digital deformationmeasurement values ascertained at different points in time can begenerated based on the at least one deformation measurement signal, inorder to approximate a time curve of the deformation of the vibratorybody.

What is claimed is:
 1. A method for ascertaining density, ρ, of a fluidcontacting an oscillatably held, vibratory body, especially a metalvibratory body, which can be excited to execute vibrations, wherein thevibratory body has a specific thermal conductivity, especially onegreater than 5 W K⁻¹ m⁻¹, and, thus, a thereon dependent, thermalconductivity, effective for heat transfer from, on the one hand, a fluidcontacting, first surface of the vibratory body, which has a fluidtemperature, namely a temperature of the fluid contacting the firstsurface, to, on the other hand, a non fluid contacting, second surface,as well as having a heat capacity, and wherein a temperature, of thevibratory body, namely a temperature of the vibratory body dependent onthe fluid temperature, is variable, the method comprises the steps of:causing the vibratory body contacted by fluid to vibrate in such amanner that it executes, at least partially, resonant oscillations,namely mechanical oscillations with a resonant frequency, dependent onthe density of the fluid contacting the first surface of the vibratorybody as well as also on the temperature, of the vibratory body;registering vibrations of the vibratory body for producing at least oneoscillation measurement signal, which has at least one signal componentwith a signal frequency corresponding to the resonant frequency andconsequently dependent on the density of the fluid; applying atemperature sensor thermally coupled with the vibratory body via itssecond surface for producing a temperature measurement signal,representing a time curve of a temperature of the vibratory body, namelya temperature of the vibratory body dependent on a temperature of thefluid contacting the vibratory body on its first surface; and producinga measured value of density, based on the oscillation measurement signalas well as the temperature measurement signal during a change of thetemperature of the vibratory body, especially a change of thetemperature of the vibratory body resulting from a change of thetemperature of the vibratory body on its first surface, in such a mannerthat a discrepancy, especially a time dependent discrepancy, occurringduring the producing of the measured value of density between the timecurve of the temperature of the vibratory body and the temperaturemeasurement signal, is taken into consideration, especially in such amanner that said discrepancy, is at least partially compensated,wherein: the temperature measurement signal, especially as a result ofthe thermal conductance, and the heat capacity, of the vibratory body,follows a change of the temperature of the vibratory body from abeginning first temperature value, to a second temperature value, onlytime delayed, a change resulting especially from a change of thetemperature of the fluid contacting the vibratory body on its firstsurface and/or from a fluid change; and so that the temperaturemeasurement signal, corresponds consequently to said second temperaturevalue, only time delayed.
 2. The method as claimed in claim 1, furthercomprising the step of: applying the oscillation measurement signal forproducing a measured value, of frequency representing the resonantfrequency of the vibratory body contacted by the fluid; applying thetemperature measurement signal for producing a measured value, oftemperature representing the temperature of the vibratory body; andapplying both the measured value, of frequency as well as also themeasured value, of temperature for producing the measured value ofdensity.
 3. The method as claimed in claim 1, further comprising thestep of: producing a sampled sequence, of frequency, namely a sequenceof digital frequency values ascertained at different points in timebased on the at least one oscillation measurement signal, wherein thesampled sequence, approximates a time curve of the resonant frequency ofthe vibratory body.
 4. The method as claimed in claim 2, furthercomprising the step of: applying the sampled sequence of frequency forproducing the frequency measured value.
 5. The method as claimed inclaim 3, further comprising the step of: applying the sampled sequenceof frequency for producing a delayed sampled sequence, of frequency,namely a sequence of digital frequency values ascertained at differentpoints in time based on the sampled sequence of frequency, in order toapproximate the time curve of the resonant frequency of the vibratorybody, in such a manner that said delayed sampled sequence of frequencymore slowly approaches a time curve of the resonant frequency followingon a change of the resonant frequency, especially a jump-like change ofthe resonant frequency, than the sampled sequence of frequency.
 6. Themethod as claimed in claim 5, further comprising the step of: applyingthe delayed sampled sequence of frequency for producing the frequencymeasured value.
 7. The method as claimed in claim 1, further comprisingthe step of: producing a sampling sequence, of surface temperaturenamely a sequence of digital temperature values ascertained at differentpoints in time based on the at least one temperature measurement signal,in order to approximate a time curve of the temperature on the secondsurface of the vibratory body.
 8. The method as claimed in claim 1,further comprising the step of: producing an estimated sequence oftemperature of the vibratory body, namely a sequence of digitaltemperature values ascertained at different points in time based on theat least one temperature measurement signal, in order to approximate atime curve of the temperature of the vibratory body, in such a mannerthat said estimated sequence of temperature of the vibratory body morequickly approaches a time curve of the temperature of the vibratory bodyfollowing on a change of the temperature on the second surface of thevibratory body, especially a jump-like change and/or a change resultingfrom a change of the fluid temperature, than the temperature measurementsignal.
 9. The method as claimed in claim 7, further comprising the stepof: applying the sampling sequence of surface temperature for producingthe estimated sequence of temperature of the vibratory body, in such amanner that the estimated sequence of temperature of the vibratory bodymore quickly approaches a time curve of the temperature of the vibratorybody following on a change of the temperature on the second surface ofthe vibratory body, especially a jump-like change and/or a changeresulting from a change of the fluid temperature, than the samplingsequence of surface temperature.
 10. The method as claimed in claim 8,further comprising the step of: applying the estimated sequence oftemperature of the vibratory body for producing the measured value oftemperature.
 11. The method as claimed in claim 7, further comprisingthe step of: applying the sampling sequence of surface temperature forproducing the measured value of temperature.
 12. The method as claimedin claim 7, further comprising the step of: applying both the samplingsequence of surface temperature as well as also the estimated sequenceof temperature of the vibratory body for producing the measured value oftemperature.
 13. The method as claimed in claim 7, further comprisingthe step of: applying a digital filter differentiating the samplingsequence of surface temperature, in order to produce the estimatedsequence of temperature of the vibratory body, especially a digitalfilter designed as a high pass digital filter of first or higher orderand/or a digital filter embodied as a FIR filter.
 14. The method asclaimed in claim 8, further comprising the step of: applying a filterdifferentiating the temperature measurement signal, in order to producethe estimated sequence of temperature of the vibratory body, especiallya filter designed as a high pass filter of first or higher order and/oran active filter.
 15. The method as claimed in claim 5, furthercomprising the step of: applying a digital filter integrating thesampled sequence of frequency, in order to produce the delayed sampledsequence of frequency, especially a digital filter designed as a lowpassfilter of first or higher order and/or embodied as a FIR filter.
 16. Themethod as claimed in claim 1, further comprising the step of: applying astrain sensor mechanically coupled with the vibratory body via itssecond surface for producing a deformation measurement signalrepresenting a time curve of a deformation of the vibratory body, namelya deformation of the vibratory body dependent on the temperature of thevibratory body and/or a force acting on such.
 17. The method as claimedin claim 16, further comprising the steps of: producing a samplingsequence of deformation, namely a sequence of digital deformationmeasurement values ascertained at different points in time based on theat least one deformation measurement signal, in order to approximate atime curve of the deformation of the vibratory body; and applying thesampling sequence of deformation for producing the measured value ofdensity.
 18. The method as claimed in claim 1, wherein: the vibratorybody is an oscillatably held, measuring tube having a lumen surroundedby a tube wall, especially a tube wall of metal.
 19. The method asclaimed in claim 18, wherein: the measuring tube is adapted to beimmersed in fluid; and the first surface of the vibratory bodycontacting the fluid is formed by an outer surface of the tube wall andthe non fluid contacting, second surface of the vibratory body by aninner surface of the tube wall facing the lumen.
 20. The method asclaimed in claim 18, further comprises the step of: allowing fluid toflow in the lumen, so that the fluid contacts the inner surface of thetube wall, wherein: the measuring tube is adapted to carry fluid,especially flowing fluid; and the first surface of the vibratory bodycontacting the fluid is formed by an inner surface of the tube wallfacing the lumen and the non fluid contacting, second surface of thevibratory body by an outer surface of the tube wall,
 21. The method asclaimed in claim 20, further comprising the step of: allowing fluid toflow in the lumen, so that the fluid contacted the inner surface of thetube wall.
 22. The method as claimed in claim 1, wherein: the vibratorybody is adapted to carry fluid, respectively to be flowed through byfluid.
 23. The method as claimed in claim 1, wherein: the vibratory bodyis adapted to be immersed in fluid, or flowed on by fluid.
 24. Themethod as claimed in claim 1, wherein: the vibratory body comprises anoscillatably held membrane; and the first surface of the vibratory bodycontacting the fluid is formed by means of a first membrane surface andthe non fluid contacting, second surface by a second membrane surfacelying opposite the first membrane surface.
 25. The method as claimed inclaim 24, wherein: the vibratory body further comprises a paddle affixedon the first membrane surface for protruding into fluid.